It is highly desirable to be able to produce pure amines and polyamines by the reaction of acrylonitrile with alcohols or amines. While the reaction of the amines and alcohols and other nucelophilic species with acrylonitrile proceeds, another competitive reaction, namely free radical polymerization also occurs to varying extents. This results in polyacrylonitrile adducts and lower amounts of acrylonitrile added to the amine or alcohol. The problem is worse when alcohols, amines or polyoxyalkylene glycol raw materials are solid at reaction temperatures. The increasing of reaction temperatures to obtain liquid reactants, results in a major increase of polyacrylonitrile produced. This has resulted in the cyanoethylation reaction with acrylonitrile being of limited use for either high viscosity, high molecular weight or solid reactants. The current invention overcomes these limitations by allowing for the use of elevated temperatures during reaction and little or no polymerization of acrylonitrile. The polymerization reaction is effectively stopped, even at elevated temperatures, by the addition of a specific class of free radical inhibitors. As previously stated, the products prepared by this novel process will have fewer undesirable by products giving lighter color, higher amine values, higher primary amine content, lower hydroxyl values which are indications of the greater reaction efficiencies. The process is shorter in duration and substantial reduction in catalyst poisoning in the hydrogenation step. This process with its inherent lower polyacrylonitrile content allows for the elimination of a washing step practiced in the older processes, prior to hydrogenation. ##STR1##
It is known that acrylonitrile is very reactive and will polymerize easily. In cases were it is desirable to react the acrylonitrile in a free radical reaction, this polymerization results in loss of reactive raw material and undesired by products. Prior to the compositions of the present invention, low temperature of reaction was used to minimize the rate of homopolymerization of acrylonitrile. This results in two drawbacks. First not only does the rate of homopolymerization slow, but the rate of the desired reaction also slows. Second, there is a limitation on the type of reactant which can be reacted. Only low viscosity materials react effectively to give the desired nucleophilic adduct. Materials which are solids, high molecular weight or viscous liquids at the lower reaction temperatures cannot be used in the process. This imposes a practical limit on the type of product which can be cyanoethylated using acrylonitrile.
We have found that the addition of small amounts of stable free radical compounds to the starting amine or alcohol prior to commencing the reaction to make the ether amine or diamine, effectively eliminates the free radical polymerization of the acrylonitrile and has no effect upon the desired cyanoethylation (reaction sequence 1). Reaction sequences can now be conducted at higher temperatures allowing for cyanoethylation of materials heretofore impossible or difficult to react prior to the compositions of the present invention.
Reaction conditions, rates and purity are all major concerns when reproducible high purity amines or ether amines are desired. The use of stable free radical compounds has surprisingly been found to be effective in preventing the undesired free radical polymerization of acrylonitrile.
A variety of "standard antioxidants" have been shown to inhibit or retard vinyl polymerization. The commonly used inhibitors appear to function by reacting in some manner with an initiator radical to yield a species of lower reactivity that results in a lower tendency to continue chain propagation. These "standard antioxidants", which are ineffective in our invention, include phenols, quinones, aromatic nitro and nitroso compounds, amines and thiol compounds.
Stable free radicals have been known for many years and exists in the patent literature, but have been used primarily, if not exclusively in the prevention of polymerization in vinyl reactive systems U.S. Pat. No. 4,670,131 issued to Ferrell discloses the use of stable free radical compounds in the prevention of polymerization of olefinic materials. This process relates to the reaction of a vinyl containing material to make a saturated higher molecular weight species. Clearly, this reaction has been understood in terms of free radical chemistry. Free radical inhibitors or scavengers inhibit or retard the polymerization of chain propagating reactions in vinyl monomer systems. The mechanism is thought to be a scavenger of the free radicals which form on the monomer. Since the free radical scavenger is stable, it reacts rapidly with free radicals of the monomer, which form in low concentrations.
In addition to abstraction, several compounds retard vinyl polymerization by radical addition which again produces a radical species which is not reactive toward the monomer. Quinone type inhibitors are probably the best example of this kind of inhibition mechanism.
It is appreciated that the free radical reaction process is not inhibited by the typical vinyl inhibitors like phenols, quinones, aromatic nitro and nitroso compounds, amines and thiol compounds. We have discovered that in order to develop an inhibitor for acrylonitrile reactions, a different type of approach must be employed. Stable free radicals, such as NOVA INHIBITOR 469, (a nitroxyl type stable free radical) provide such a system. These stable free radicals are far too stable to initiate polymerization, but their free electron is available to immediately react with any radical initiator, rendering the potential initiator totally inactive. This class of inhibitors has been found to be effective in preventing acrylonitrile polymerization, a free radical type reaction, but has no effect upon the desired nucleophilic reaction.
The preferred stable free radical is a nitroxyl.